This invention relates to a torque limiter for selectively coupling and decoupling two rotary members according to the torque applied.
FIG. 2 shows a conventional torque limiter. An outer cylinder 21 is rotatably mounted around an inner cylinder 20. A coil spring 22 is provided between the cylinders.
The coil spring 22 is formed from a wire having a square section, and has a small-diameter portion 22a and a large-diameter portion 22b. The small-diameter portion 22a and the large-diameter portion 22b comprise a continuous wire having helical turns 22c and 22d having their adjacent turns in close contact with each other.
The small-diameter portion 22a has its inner periphery 22e pressed against the outer periphery 20a of the inner cylinder 20. The outer periphery 20a is smoothly finished with its roughness Ra (arithmetical mean deviation) being about 0.07. Grease is applied to the portion of the outer periphery 20a adapted to be brought into contact with the inner periphery 22e to form a lubricating oil film between the inner periphery 22e and the outer periphery 20a.
The coil spring 22 has an engaging portion 23 on one end thereof, i.e. on the end of the small-diameter portion 22a, and has an engaging portion 24 on the other end, i.e. the end of the large-diameter portion 22b. The engaging portion 23 is coupled to one end portion 25 of the outer cylinder 21, whereas the engaging portion 24 is coupled to a closure member 27 fitted in the other end 26 of the outer cylinder 21.
The maximum torque that can be transmitted between the inner cylinder 20 and the outer cylinder 21 (hereinafter referred to as "limit torque") is determined by the force with which the small-diameter portion 22a is tightened against the inner cylinder 20 and the friction coefficient between the inner periphery 22e and the outer periphery 20a.
As the rotating speed of the inner cylinder 20 increases, the slip speed between the small-diameter portion 22a and the outer periphery 20a correspondingly increases due to the increasing difference in rotating speed between the inner cylinder 20 and the outer cylinder 21. As shown in FIG. 3, this causes the lubricating condition in the area between the inner periphery 22e and the outer periphery 20a to change from boundary lubrication to a mixture of fluid lubrication and boundary lubrication, resulting in a reduced friction coefficient between the inner periphery 22e and the outer periphery 20a. The limit torque thus drops.
Further, since the helical turns 22c forming the small-diameter portion 22a have their sides in close contact with each other, it is difficult for grease to enter the space between the inner periphery 22e and the outer periphery 20a, so that the outer periphery 20a tends to wear quickly, causing fluctuations in the limit torque especially at the beginning of use.
An object of this invention is to provide a torque limiter which keeps a high limit torque at a high rotating speed and, which suffers no fluctuation in limit torque at the beginning of use.